This is a great way to transform a pair of sweats that may be a bit short or unflattering into track pants. I personally find this style more flattering with running shoes as well…which I wear a lot. You can also do this with sweatpants your littles have started to out grow by adding a couple of inches in length. These make for great stylish play pants and they don't get dirty and ripped at the hem.
If you're trying to lengthen the pants like your kids, then don't cut and sew the band right on the hem.
Last sew the inner seam up including the ribbing. I start from the ribbing and gradually met with the original seam. Cut any excess seam allowance.